The invention relates to a method of making fibrids from thermoplastics such as polyolefins, polyvinyl halides, polyvinylidene halides, other polymers on the basis of vinyl derivatives, as, for instance, polyvinyl esters, polyvinyl pyrrolidone, polyvinyl lactam, polyamides, polyesters, polyacrylonitrile, polyurethanes and their copolymers and the like, by expansion evaporation, in the presence of a gaseous propellant, of a corresponding polymer solution under elevated pressure and elevated temperature in a two-substance mixing nozzle under shear-inducing conditions, as well as the application of a process of the stated kind.
The term "fibrids", as used herein, is to be understood to refer to fibers which are highly oriented in the longitudinal direction and have a cellulose-like structure of up to several millimeters in length (cf. Angewandte Chemie, 90, 833/1978).
Various procedures are known for the preparation of fibrids from thermoplastics, and they differ in the manner of energy input for the work to be performed in forming fibrids from a liquid phase, e.g., by shear with cooling, precipitation or vaporization of the liquid phase.
Therefore, in the case of shear-induced crystallization, fibrids of polyethylene are formed under the action of turbulent vortices by the cooling of an approximately 5 wt-% solution of polyethylene in a solvent.
Fibrids can also be formed by the precipitation of the polymer from a solution by means of a liquid serving as precipitating agent (DE-AS No. 1,469,120) or by cooling under the shearing action of rotating elements or of a turbulent stream of liquid.
Another method of preparing fibrids sets out from a polymer film stretched along one axis. By mechanical comminution in an appropriate apparatus, the use of a precipitating agent is rendered unnecessary (U.S. Pat. No. 3,693,851).
In the preparation of fiber-like products from the solution of a polyolefin by expansion vaporization of the solvent (DE-AS No. 2,227,021), the energy required for the production of the shearing forces and for the vaporization of the solvent is taken from the melting heat of the polymer and from the system by expansion through a throttle valve of a solution of the polymer, which is under elevated pressure and elevated temperature.
The disadvantage of this method consists in the necessity, for the evaporation of the solvent and the achievement of the required high velocities of more than 50 ms.sup.-1 in the liquid phase ahead of the throttle valve, of a high pressure and a temperature so high that it is often higher than the melt temperature or molten solution temperature, as the case may be, of the dissolved polymer.
Particularly in the case of polymers sensitive to degradation, such as polyvinylidene fluoride or even polyvinyl chloride, for example, there are limits to the use of the above-described method above the melting temperature of the polymer or the molten solution temperature.
Another limitation of the expansion vaporization process is to be seen in the fact that the rate of vaporization and the velocity of flow are established by the temperature and pressure of the polymer solution. As a result, the dimensions of the fiber-like particles thus obtained, such as their length, for example, have to be reduced by subsequent operations, such as grinding.
In a modification of the expansion vaporization process, gaseous nitrogen is additionally used as the propellant, and the evaporation is produced in a two-substance mixing nozzle (cf. G.B. Pat. No. 1,491,050 relating to the production of fibrids from polyolefins).
It is the object of the invention to further improve the expansion vaporization process such that true fibrids can be produced by thermoplastic substances, even from polymers, such as PVDF, which are sensitive to degradation at temperatures only slightly above the melting point, without the need to add mechanical comminution processes. to arrive at fibrids and porous structures prepared therefrom. These fibrids are characterized by a large specific surface area and other desired properties.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects obtained by its use, reference should be had to the accompanying drawing and descriptive matter in which there is illustrated and described a preferred embodiment of the invention.